Johnston served as city commissioner of Key West, Florida for eight years before running for mayor in 2018.

[2] She defeated Margaret Romero in the general election, receiving 6,635 votes.

She is the first openly lesbian woman to be elected mayor of a Florida city.

[3] Additionally, she is the first woman to serve as mayor of Key West since Sheila Mullins' term in the 1990s.

She was sworn in to office on November 19, 2018, succeeding Craig Cates.